Catalyst regeneration apparatus

ABSTRACT

A catalyst regeneration apparatus for the oxidation of coke from a spent catalyst, said coke being converted to CO, and for the conversion of the CO to CO 2 . Hot regenerated catalyst is recycled from a dense bed in the regeneration zone to mix with incoming spent catalyst in a mixer zone. The mixer zone operates in dense phase and is supplied with a relatively small amount of a fluidizing medium, preferably air. After the mixing of spent and fresh catalyst is substantially completed, a relatively large amount of a regenerating gas, preferably air, is admixed with the catalyst mixture, and some coke oxidation occurs. The balance of coke oxidation takes place in a downstream-situated regeneration zone of substantially conventional design. There is a transfer section connecting the mixer zone to the regeneration zone through which the relatively large amount of the regenerating gas is admitted. The transfer section is preferably a frustoconical surface, the horizontal cross section of smallest perimeter of which adjoins the outlet of said mixer zone, and regenerating gas addition is made at the base of the transfer section in order to provide a venturi effect which ensures good air-catalyst mixing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of our prior,copending application Ser. No. 908,301 filed May 22, 1978 and issued asU.S. Pat. No. 4,197,189, on Apr. 8, 1980 which application isincorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of art to which this invention pertains is catalystregeneration. The present invention will be most useful in a process forregenerating spent FCC catalyst, but should be useful in any processwherein coke is burned from a solid, particulated, fluidizable catalyst.

2. Description of the Prior Art

Much of the world's crude oil is subjected to fluid catalytic cracking(hereafter FCC) processes to convert the heavier material into lighterproducts. The fluid catalyst used in these processes is quicklycontaminated with coke, and to permit the reuse of the catalyst in theprocess the coke must be burned from the catalyst. Thus there is usuallyassociated with every FCC process unit a fluid catalyst regenerator.

In the past, the catalyst regenerators have burned coke from thecatalyst by adding air to a single regeneration zone. Coke was burned toprovide a mixture of carbon monoxide and carbon dioxide. Regenerationwas usually incomplete, because adding the stoichiometric requirement ofair to the catalyst regeneration zone invariably resulted inconsiderable oxidation of carbon monoxide to carbon dioxide, usually inan upper portion of the regeneration vessel where no catalyst wasavailable to act as a heat sink. This burning of carbon monoxide tocarbon dioxide, often called "afterburning", resulted in extremely hightemperatures which could damage the regenerator, hence air addition wasrestricted to protect the apparatus.

Recently, there have been attempts made to promote combustion of COwithin the regenerator to recover the heat liberated by this combustion,to permit use of this heat in the FCC process and to permit morethorough regeneration of the catalyst.

Examples of these recent regeneration processes are Stine et al U.S.Pat. No. 3,844,973 and Horecky, Jr. et al U.S. Pat. No. 3,909,392, theteachings of both of which patents are incorporated by reference.

In the catalyst regeneration apparatus of Conner et al, U.S. Pat. No.3,893,812 (U.S. Class 23/288 B), the teachings of which are incorporatedherein by reference, an improved regenerator design is disclosed. Afirst intermediate density zone or combustor, i.e. a zone containingfluid catalyst of intermediate density, receives spent catalyst and air,permitting most of the coke to be burned therein. Catalyst andregeneration gases, and CO produced during coke combustion, are thenpassed upwardly through a dilute-phase transport riser whereinsignificant amounts of the CO are burned to CO₂. Finally the regeneratedcatalyst is collected in a second dense bed. This process provides forrecycle of a portion of the hot regenerated catalyst from the seconddense bed to the combustor via an external catalyst recycle means. Thefunction of catalyst recycle is to increase the temperature in thecombustor and increase the rate of coke oxidation. It is also known toprovide for internal catalyst recirculation from the second dense bed tothe combustor.

Another example of a process operating with two dense beds of catalyst,separated by a dilute phase transport riser, is German OS No. 25 26 839,corresponding to U.S. Ser. No. 479,726, filed June 17, 1974 (Class 252),the teachings of which are incorporated by reference. In this referencehot regenerated catalyst from the second dense bed is admixed with spentcatalyst from the FCC reactor in a riser beneath the first dense bed orcombustor of the FCC regenerator. Dilute-phase conditions are maintainedin the riser (item 34 in the drawing of the U.S. application) by theaddition of sufficient air. The dilute-phase condition is indicated onthe drawing, and would also be expected as most FCC technologists designrisers for dilute-phase conditions.

We have discovered that significantly improved operation is possible byseparating and optimizing the desired operations which occur at theinlet to the catalyst regenerator. Refiners are trying to do two things,to mix hot regenerated catalyst with relatively cooler spent catalyst,and also to mix spent catalyst with air. The former ensures that thecatalyst is supplied to the combustor at a uniform temperature with auniform carbon distribution, and the latter ensures that there is auniform supply of oxygen. These requirements must be met if uniformregeneration of the catalyst is to be achieved. Conditions which areoptimum for good catalyst-catalyst mixing are not optimum for promotinggood catalyst-air mixing.

Our inventive concept provides for a riser-mixer containing a relativelydense-phase, turbulent catalyst-catalyst mixer zone, acatalyst-regeneration gas mixing zone, situated above said riser and acombustor zone situated above the catalyst-regeneration gas mixing zone.The catalyst-regeneration gas mixing zone is often referred to herein asa transfer section or transition zone, this being based upon the factthat the catalyst-regeneration gas mixing zone provides a transitionfrom the lower mixer zone to the upper combustor. We have discoveredthat by providing zones for each of the catalyst-catalyst andcatalyst-regeneration gas mixing steps a significantly improved catalystregeneration is made possible.

Dense-phase, turbulent conditions are maintained in the mixer zone byseverely limiting the amount of fluidizing gas which is added to thelowermost portion thereof. Air is preferably used as the fluidizingmedium, because it is cheap and readily available and its presencepermits some combustion to occur, though it is not essential to use airas the fluidizing medium. A relatively-small-diameter riser isnecessary, the riser having a diameter typically one-fourth that of thecombustor, to promote intimate mixing of regenerated and spent catalystin the riser. A significant amount of combustion air is added at thetransfer section between the riser and the combustor to promote cokeburning.

Intimate mixing of regenerated and spent catalyst occurs in the smalldiameter, dense bed mixer zone. The spent catalyst is heated by hotregenerated catalyst, so that when spent catalyst contacts combustionair, coke burning readily occurs. In a preferred embodiment thetransition zone or transfer section between the mixer zone and thecombustor is in the shape of a frustum of a cone or of a funnel, thehorizontal cross section of smallest area of which adjoins the mixerzone, wherein the bulk of combustion air is added to the transfersection through holes distributed about the surface of the funnel. Whenthe angle between the center line of the mixer zone and the surface ofthe funnel is about 45°, fabrication costs are minimized and a venturiacceleration effect is obtained which provides for excellent air andcatalyst mixing. Good results can, however, be obtained with otherangles. The transition section may comprise a substantially horizontalsurface whose outside perimeter is intermediate the outside perimetersof the chamber which comprises the combustor zone and the mixer zone,said horizontal surface containing multiple perforations connective withthe regeneration gas inlet, and the mixer zone discharging upwardly pastthe substantially horizontal surface and into the chamber.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a regenerationapparatus for regenerating spent catalyst with a regeneration gas whichcomprises in combination: (a) a vertical mixer zone having at the lowerportion thereof a spent catalyst inlet, and having at the upper portionthereof an outlet for a mixture of spent and regenerated catalyst; (b) acatalyst receiving chamber for containing a relatively dense-phasefluidized bed of catalyst, said chamber having at least twice thediameter of said mixer zone and having at the bottom thereof an inletmeans for receiving a mixture of spent and regenerated catalyst, andhaving a regenerated-catalyst and spent regeneration gas outlet means atthe top portion of said chamber whereby catalyst and regeneration gaspass in admixture out of said chamber; (c) a transfer sectionintermediate said mixer zone and said chamber discharging upwardly fromsaid mixer zone into said chamber, said transfer section containing aregeneration gas inlet at the lowermost portion of the same and abovesaid spent catalyst, said regenerated catalyst and said fluidizinginlets of step (a); (d) a chamber outlet connected to the upper portionof said chamber for removal of regenerated catalyst and spentregeneration gas; (e) a regenerated catalyst receiving zone incommunication with said chamber outlet, said receiving zone containing aspent-regeneration-gas outlet means for the withdrawal of spentregeneration gas from the regeneration apparatus, a regenerated-catalystoutlet means for the withdrawal from the regeneration apparatus of aportion of regenerated catalyst and a regenerated-catalyst recycleconduit connected to said mixer zone by means of a regenerated catalystinlet for the passage of a portion of regenerated catalyst from saidreceiving zone through said conduit and said inlet to said mixer zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 describes a general embodiment of the apparatus of our invention.

FIG. 2 is an enlarged side view of a preferred embodiment, wherein thetransfer section comprises a frustoconical surface, the horizontal crosssection of smallest perimeter of which adjoins the outlet of the mixerzone.

FIG. 3 is an enlarged side view of an embodiment wherein the transfersection comprises a substantially-horizontal surface whose outsideperimeter is intermediate the outside perimeters of the chamber and themixer zone.

FIG. 1 shows a side view of an embodiment of our invention, whichbasically comprises spent catalyst receiving chamber 1, also designatedherein as a combustor, regenerated catalyst receiving chamber 2,transfer conduit 3, mixing conduit 4, transfer section consisting ofventuri conduit 5, and regenerated catalyst recycle conduit 6. Thecombustor, or spent catalyst receiving chamber 1, is a chambercontaining a relatively dense-phase fluidized bed of catalyst whereinthe majority of coke is oxidized. Transfer conduit 3 connects combustor1 with regenerated catalyst receiving chamber 2. The regeneratedcatalyst receiving chamber 2 separates regenerated catayst from fluegas, and contains a dense bed of regenerated catalyst.

The mixing conduit 4 is a vertical mixer zone for mixing of spentcatalyst, hot regenerated catalyst and a limited amount of fluidizingmedium, preferably air. Venturi conduit 5 promotes mixing of catalystwith air.

Spent catalyst from an FCC unit, or any other catalytic unit whereincoke is deposited on a solid, particulated catalyst, is charged via line9, valve 33 and line 28 into the lower portion of mixer zone 4. Hotregenerated catalyst is recycled from the regenerated catalyst receivingchamber via line 6, valve 38, and line 39 into the lower portion ofmixer zone 4. A small portion of fluidizing air from line 34 is added tothe lower portion of mixer zone 4 via distributor 44. Only a minorportion of the total air required for combustion is added via line 34.

Mixing of the spent and regenerated catalyst, and, to a limited extent,coke oxidation occur in mixer zone 4. Once the regenerated and spentcatalysts are mixed together additional combustion air, preferablysufficient to permit complete oxidation of all of the coke on thecatalyst, is added via distributor 40 which receives combustion air fromline 41. Venturi section 5 promotes mixing of combustion air withcatalyst in combustor 1. It is not essential to have a venturi section 5as a transfer section, but use of such a venturi section promotesuniform mixing of combustion air with catalyst, and also promotesfurther mixing of spent and regenerated catalyst. The spent andregenerated catalyst have already been fairly well mixed by the timethey reach the level of air distributor 40, but additional mixing andcontact of hot regenerated catalyst and spent catalyst is stilldesirable. Most of the coke is burned from the spent catalyst within theintermediate density bed 7, to produce substantially regeneratedcatalyst which leaves combustor 1 from region 8 via outlet 11 located atthe top of the combustor. Outlet 11 is also the inlet to the dilutephase transport riser 3. Catalyst in riser 3 is in dilute-phase bed 13.

Regenerated catalyst is removed from the riser 3 via outlet means 12which is connected at a separation means whereby catalyst and spentregeneration gas are separated. Separation means which can be usedinclude a disengaging space, as shown in the drawing, or a cycloneseparator, or combinations of these. In the embodiment shown in thedrawing a disengaging space 14 and cyclone separator 16 are used incombination to separate catalyst from gas. The function of cap 15 at theupper end of riser 3 is to provide a buffer space so that catalyst willnot erode the upper end of conduit 3.

Regenerated catalyst receiving chamber 2 contains dilute phasedisengaging space 14 and dense-phase catalyst bed 17. Regeneratedcatalyst passes downwardly from outlet means 12 into dense bed 17separated from the dilute phase by interface 18. Spent regeneration gasenters cyclone 16 via inlet 19. Substantially catalyst-free gas isremoved from cyclone 16 via outlet 21 and passed through line 22 andoutlet 23 into plenum chamber 24, and is eventually removed from thesystem via outlet 25. Regenerated catalyst is removed from cyclone 16via dip leg 20. Regenerated catalyst is accumulated in dense bed 17which covers the entire lower space of chamber 2. A portion of hotregenerated catalyst is withdrawn via line 6, valve means 38, and line39 for addition to mixer zone 4 as previously discussed. Another portionof hot regenerated catalyst is withdrawn via line 30 and valve means notshown to the FCC reactor. It is possible to withdrawn catalyst forrecycle to the mixer zone 4 from line 30, or separate means may beprovided, as shown, for obtaining catalyst for recycle to the reactorand to the mixer zone.

The flow rate of regenerated catalyst to the riser mixer and the reactorwill usually be controlled by slide valves. It is possible to use othermeans to control catalyst flow, such as providing for recycle of hotregenerated catalyst to mixer zone 4 via a number of open conduits atdifferent elevations in the dense bed of regenerated catalyst. A higherlevel of catalyst will force more catalyst flow because of increasedfluid head. Most refiners prefer the precise control which a slide valveprovides, but it is not absolutely necessary.

Similarly, the design shown in FIG. 1 is an excellent design for use inconjunction with an FCC unit wherein complete combustion of CO to CO₂ isdesired. It is not necessary to operate with complete afterburning ofCO, and there may be situations where such complete afterburning must beavoided either because of limits of temperature which can be experiencedwithin the regeneration zone, or perhaps because a refiner has anexisting CO boiler which must be used. Even when complete combustion ofCO is not required, it is still desirable to have mixing of hotregenerated catalyst with incoming spent catalyst, and the practice ofthe present invention will permit a more uniform regeneration ofcatalyst to occur.

The importance and size of the dilute phase transport riser 3 havediminished in recent years. There is a trend among refiners to use aCO-burning promoter. When a promoter is used, very complete combustionof carbon monoxide occurs within combustor 1, and riser conduit 3primarily serves to transfer catalyst from the combustor to theregenerated catalyst receiving chamber and to effect heat exchangebetween gas and catalyst. The benefits of the present invention, i.e.,better mixing of hot regenerated catalyst with relatively cooler spentcatalyst, will be useful whether or not a CO promoter is used. When aCO-combustion promoter is used, it may either be incorporated directlyinto the catalyst during manufacture, or added to the regenerator in theform of a solid or a liquid solution. The particular type ofCO-combustion promoter used is not critical, and forms no part of thepresent invention.

FIG. 2 is an enlarged view of a preferred embodiment of the presentinvention with a mixer zone 4 and a transfer section comprising venturisection 5 leading to the lower portion of combustor 1. The venturisection comprises a frusto-conical surface the horizontal cross sectionof smallest area of which adjoins the outlet of mixer zone 4. The angleθ indicated in the drawing should be about 45° to provide an optimumventuri effect. This venturi effect is desirable, but not absolutelyessential.

FIG. 2 also shows the use of an air box to add the primary air supply tothe regeneration zone. In this embodiment sealed container 43encompasses the lower portion of venturi section 5 and serves as an airbox. Combustion air is added via line 41 to air box 43. Perforations 42in the sidewalls of venturi section 5 permit combustion air to be addedto the mixed catalyst from mixer zone 4. One of the advantages of thisconstruction is that there is no obstruction of the interior of mixerzone 4. It is still preferred to maintain a venturi section by providingan angle of roughly 45° between the center line of the mixer zone andthe funnel side, but it is not essential to do so. It would beacceptable to use an air box to add combustion air through perforationsin the wall of the upper part of mixer zone 4.

FIG. 3 is an enlarged view of another embodiment of the presentinvention in which the transfer section comprisessubstantially-horizontal surface 45. The outside perimeter of surface 45is intermediate the outside perimeter of chamber 1 and mixer zone 4.Sealed container 43 encompasses an upper portion of mixer zone 4 whichdischarges upwardly past surface 45 into chamber 1. Combustion air isadded via line 41 to sealed container or air box 43. Perforations 46 insurface 45 permit combustion air to be added to the mixed catalyst frommixer zone 4. All other reference numerals in FIG. 3 and the items towhich they refer are as set forth in the above description of FIG. 2.

These figures are meant to be illustrative and not limiting.

DESCRIPTION OF THE INVENTION

Reference will now be directed to the process aspects of our invention.To assist in understanding, a number of terms will be briefly defined.

The FCC process contacts a hydrocarbon feed with cracking catalyst in ahydrocarbon-reaction zone to produce product, spent catalyst, and coke.Coke is oxidized from the spent catalyst in a catalyst regeneration zoneto restore the catalyst activity and permit its reuse. Spent catalystmeans catalyst withdrawn from any hydrocarbon reaction zone, theactivity of which catalyst has been reduced by coke deposition thereon.Spent catalyst may contain 0.1 to 5 wt. % carbon, but typically FCCoperations produce spent catalyst with 0.5 to 1.5 wt. % carbon.Regenerated catalyst is catalyst from which most of the coke has beenremoved by oxidation in a regeneration zone. FCC catalyst regenerated bythe process of our invention will typically contain about 0.01 to 0.2wt. % carbon, and usually about 0.01 to 0.1 wt. % carbon. Coke comprisesa mixture of carbon and hydrogen deposited on catalyst during itsattendance at sites of hydrocarbon conversion reactions. Most of thecoke is carbon, but coke can contain from 5 to 15 wt. % hydrogen. Thecoke content of spent catalyst is almost, but not exactly, equal to thecarbon content of a spent catalyst.

Regeneration gas is any gas which contacts catalyst within theregeneration zone. Fresh regeneration gas includes air or oxygenenriched or deficient air. Coke can be oxidized to produce spent orpartially-spent regeneration gas. The regeneration gas is"partially-spent" when it contains a reduced concentration of freeoxygen as compared to fresh regeneration gas. The CO concentration inpartially-spent regeneration gas may range from 0.1 to 15 mole percent,and typically will be 5 to 14 mole percent. Spent regeneration gas has areduced CO content, compared to partially-spent regeneration gas.Preferably, spent regeneration gas contains less than 1000 ppm CO andtypically less than 500 ppm CO. The term "essentially completecombustion of CO" means the CO concentration in spent regeneration gashas been reduced to less than 1000 ppm, preferably less than 500 ppm.

A brief consideration of the design and operation of typical prior artregeneration processes will make the operation and advantages of ourprocess more apparent. In the prior art process, especially in theprocess described in German OS No. 25 26 839, there is disclosed adevice consisting of a combustor or first dense bed, a dilute phasetransport riser and a second dense bed for collection of regeneratedcatalyst. There is provided for recycle of hot regenerated catalyst tothe combustor and for mixing of hot regenerated catalyst with incomingspent catalyst in a vertical riser zone upstream of the combustor. Thisriser will provide some mixing, but the mixing will not be as efficientas in our process. The reason is that in the German OS dilute phaseconditions are maintained throughout the riser. We maintain adense-phase, turbulent bed which promotes mixing. A further improvementof our design over that of the reference is provision of a venturisection at the base of the combustor to promote more intimate mixing ofcatalyst and air.

In our system the catalyst-catalyst mixing and subsequent air-catalystmixing are achieved by splitting the air into two locations. In themixer zone section 4, dense phase operation is maintained by limitingthe air velocity so that the superficial velocity is not enough totransport catalyst in the dilute phase. For typical FCC catalyst,superficial velocity required for a dense bed is normally about 0.5-3ft/sec. However when very much catalyst is present, i.e., the ratio ofweight of catalyst per volume of air is high enough, it is possible torun with a superficial velocity greater than 3 ft/sec while stillmaintaining a dense, turbulent, fluidized bed. The catalyst density willtypically be about 25 to 30 pounds/cubic foot. About 0.5 to 2.5 poundsof catalyst will be lifted up through the mixer zone for each standardcubic foot of entering gas. There is nothing novel in these fluidizationconditions, they are all within the broadly defined limits of FCCoperation, e.g., conventional FCC regenerators operating with a singledense bed of catalyst. We are not aware of any reference disclosing useof our mixer zone as a means of mixing spent and regenerated catalystupstream of a regenerator.

Spent and regenerated catalyst are mixed in this dense phase riser. Theback-mixing which occurs in a dense turbulent bed thoroughly mixes thetwo catalyst. It is best to minimize the diameter of this section of theprocess. Minimizing the diameter minimizes the distance that must betraversed by the two catalyst streams in order to achieve good mixing.The diameter of the combustor will usually be at least twice as large asthe diameter of the mixer zone.

After the catalysts are mixed, they preferably enter a second mixingstage. In this stage the remainder of the air is injected at the baseof, or perhaps throughout, a transition area or transfer section. Thetransition area may be frusto-conical, that is in the shape of a frustumof a cone, or of similar geometry which allows the catalyst andairstream to gradually spread from the mixer zone diameter to thediameter of the combustor as it ascends from mixer-zone to thecombustor. This gradual spreading provides a continuously uniformdistribution of air and catalyst which is needed for good regeneration.When maintaining the angle between the center line of the mixer-zone andthe side of the venturi section at 20° to 90°, a significant venturieffect will be achieved which will further enhance the mixing of spentand regenerated catalyst, and of catalyst with air. When a 90° angle isused, air should be added across the horizontal portion of the airdistributor separating the mixer from the combustor.

An excellent way of adding combustion air to the venturi mixing sectionis to provide an "air chest" or "air box" around the upper portion ofthe dense phase mixer zone and throughout the transfer sectionconnecting the mixer zone to the combustor.

Punched holes, or screens, or other equivalent means located around thevery top of the mixer zone section in the base of the transfer sectionwill permit easy addition of combustion air to the mixed catalyst fromthe mixer zone. One of the advantages of this method of addingcombustion air, besides its low cost and ease of fabrication, is thatthere is no obstruction of the catalyst mixture leaving the mixer zone,as would be the case to some extent if any air distributor or airsparger were placed in the path of catalyst flow from the mixer zone.

Conditions within the combustor, dilute phase transport riser, andregenerated catalyst dense bed, are all conventional. The combustortemperature will typically be 1200° to 1400° F., with a superficialregeneration gas velocity of about 3 to 10 ft/sec and a pressure ofatmospheric to 50 psig. Residence time within the combustor will usuallybe less than two minutes. Most of the coke will be oxidized in thecombustor.

In the dilute phase transport riser it was believed that most of the COpresent in the partially spent regeneration gas was burned to CO₂. It isnow believed that most CO combustion occurs in the combustor, especiallywhen a CO combustion promoter is used. The amount of CO required to beoxidized to CO₂ in the transport riser is also further reduced when thecombustor inlet system provides for uniform catalyst temperature anduniform air supply. With less efficient distribution systems in largediameter combustors one side of the zone may contain a higher percentageof regenerated catalyst. This side of the zone will therefore containless coke but will be at a higher temperature than the other side of thecombustor. As a result one side of the combustor will produce aregeneration gas which has been essentially completely oxidized, that isto say the CO will have been completely oxidized to CO₂, and thisregeneration gas will contain excess oxygen. The other side will providea regeneration gas which is deficient in oxygen and which containsunoxidized CO. The transport riser provides for the mixing ofregeneration gas from various parts of the combustor, and it allowscombustion of any residual CO that has resulted from poor mixing ofspent and regenerated catalyst or poor mixing of catalyst and air at theinlet of the combustor. Thus, with more efficient mixing systems or theuse of CO combustion promoters, or combinations of the two, theimportance and size of the dilute phase transport riser have diminished.Temperature in this zone will be about 1250° to 1450° F., with apressure slightly less than that in the combustor. Superficial gasvelocities are preferably 10 to 25 ft/sec.

Regenerated catalyst collected in the second dense bed will usually bearound 1250° to 1400° F., in a typical FCC operation. Catalysttemperatures of 1350° F. and higher are usually avoided because of thedeactivating effect of high temperatures on the catalyst. The pressurein the second dense bed, wherein regenerated catalyst is collected forrecycle to the reactor and to the combustor, will be slightly less thanthat in the combustor, slightly less because of the pressure dropassociated with getting the catalyst and gas through the system. Usuallythe catalyst is totally regenerated by the time it is in the seconddense bed, although it is possible to add additional oxidizing mediuminto this bed if desired, or to add a combustible substance, such astorch oil, to further heat up the catalyst. Such additives are notnormally necessary or desirable. It is also possible to treat theregenerated catalyst with steam, by means not shown in the drawing. ManyFCC catalysts are deactivated by such steam treatment, however, andtherefore this is not normally practiced.

The ratio of recycled freshly regenerated catalyst to spent catalyst inthe mixer zone is an is an important variable in the process. If only asmall amount of hot regenerated catalyst is recycled, there will not besufficient heat transferred to the spent catalyst. Accordingly at least25% of the material in the mixer zone of our invention should berecycled freshly regenerated catalyst. It is not normally desirable tooperate with very large amounts of recycle as such large amounts ofrecycle tend to distort the actual flow of catalyst through the systemand require that the vessels be much larger than is necessary. For thisreason the amount of hot regenerated catalyst in mixer zone 4 will notnormally exceed 80% of the catalyst inventory in this zone. For mostoperations, about a 1:1 ratio of fresh to regenerated catalyst will givegood results.

ILLUSTRATIVE EMBODIMENT

The best mode contemplated for practicing our invention is as follows;expressed in terms of the dimensions of the regeneration apparatus:

    ______________________________________                                                  Dia-  Approx.  (Approx.) Gas Super-                                           meter,                                                                              Length   ficial Velocity,                                               ft.   ft.      ft/sec                                               ______________________________________                                        Regenerated Cata-                                                             lyst Receiving                                                                Chamber     31      45       2.5                                              Combustor   20      20        6                                               Mixer zone   5      20       2.4                                              ______________________________________                                    

The above arrangement will result in approximately 3% of combustion airgoing to the mixer zone and 97% to the transition zone between the mixerzone and the combustor.

The following differences between our process and conventional ones,such as that described in the German OS with a dilute phase mixer zoneupstream of the combustor, may be pointed out:

(1) The density in our mixer zone, ranging from 10 to 40 lbs/ft³, wouldbe much greater than that in a dilute phase riser;

(2) Particle-particle heat transfer, known to take place more rapidly ina dense bed than in a dilute phase, is superior, and therefore asubstantially isothermal bed would be established very rapidly in ourmixer zone, whereas a dilute phase mixer zone would have a morepronounced temperature gradient;

(3) A dilute phase mixer zone is less efficient at distribution of spentcatalyst particles throughout the combustor vessel. Therefore, it ispossible to have localized concentrations of carbon where there is morecarbon present than there is O₂ for its combustion. The solids-mixingefficiency is much higher in a dense phase than a dilute phase, and oursystem will therefore assure that spent catalyst particles are spreaduniformly throughout the combustor, resulting in good carbon--O₂contacting and, therefore, more efficient burning;

(4) Because of more efficient heat transfer, and most of all because ofmore efficient mixing of spent and regenerated catalyst which occur inour mixer zone, we believe that a decrease in the residence timerequired in the combustor may be obtained. This could be taken advantageof by making the vessels smaller, which would save significant capitalcost of the system, and even more importantly would reduce the catalystinventory in the regeneration unit. A 10% decrease in the combustor sizewould reduce total catalyst inventory in a unit by about 5%, meaningthat the unit could operate with 5% less daily addition of new catalyst.Alternatively, a refiner could use the residence time margin afforded bythe use of the present invention to permit operation in a CO burningmode without the use of a CO burning promoter. Another way that thedecreased residence time might be used is to increase the capacity of anexisting unit without providing a larger combustor. Thus, as part of anexpansion in the capacity of a unit, our mixer zone could be addedbeneath an existing regenerator to permit increased processing capacityof spent catalyst through the regenerator, with a consequent increase inprocessing capacity in the fluid catalytic reactor.

Although maintenance of a dense-phase turbulent bed in the mixer zonepromotes excellent contact of hot regenerated catalyst with spentcatalyst, it may sometimes be desirable to incorporate additional mixingmeans into the dense-phase riser-mixer. Splitting of each catalyststream into, e.g., two streams would permit addition of spent and hotregenerated catalyst at four equally spaced radial points in the mixerzone. In a preferred embodiment, the catalyst enters the mixer zone on atangent, thus imparting a swirling motion to the material in the mixerzone. It may also be desirable to incorporate in this vessel staticmixing devices which will cause the catalyst near the edge of the mixerzone to be displaced into the center in the mixer zone. Great careshould be taken in selecting the material of construction to be used forsuch a mixer, because of the extremely abrasive conditions encounteredin such service.

It is also within the scope of our invention to operate the processusing a CO burning promoter. This promoter can be in the form of aCO-burning promoted catalyst, the promoter being incorporated in thecatalyst, or it may be in the form of a solid or liquid additive to thefeed to the process or directly to the regeneration zone. When theseadditives are used their effect will be cumulative with the beneficialeffects of the mixer zone and transfer section mixing zone of thepresent invention.

Although the present invention is especially useful in regeneratingfluidized catalytic cracking catalyst, it can also be used to regeneratecatalyst from any other process, whether fluidized or not. The processof the present invention will find great utility in regeneratingcatalyst used in converting heavy residual feed stocks, and will in factbe especially useful in these processes because of the great amounts ofcarbon deposition which occur when processing these heavy feed stocks.The present invention may also be used to regenerate catalyst fromconventional fixed bed processes, i.e., such as the reforming processwhich uses a noble metal catalyst on a solid particulated aluminasupport.

The present invention will also improve the operation of FCCregenerators of a more conventional design, i.e., those having one largedense bed wherein regeneration of catalyst occurs. In such regenerators,the single dense bed of catalyst is believed to act as a continuouslystirred tank reactor. If the reactor operated perfectly, temperaturesand compositions within the bed would be uniform. As FCC technologistsknow, however, such is rarely the case in a commercial unit as there isusually some maldistribution of catalyst and/or air. This poordistribution has been demonstrated by the color of catalyst samplestaken from conventional units which did not operate in a CO-burningmode. The regenerated catalyst had the appearance of a mixture of tablesalt and black pepper. The light-colored catalyst had been in theregenerator quite a long time and was very low in coke. The dark-coloredcatalyst had substantially escaped regeneration, and therefore had arelatively higher carbon content. Circulation of regenerated catalystand spent catalyst through a mixer zone such as contemplated by ourinvention will greatly improve the dispersion of spent catalystthroughout the regeneration zone and increase the efficiency of theoperation. Thus the improved mixing afforded by the practice of thepresent invention should improve the operation of these conventionalunits by minimizing the problems of poor catalyst distribution withinthe regeneration zone. Where our mixer zone is installed beneath a priorart regenerator containing only a single dense bed of catalyst, there isno need to install a dilute phase transport riser.

It is also possible to operate in a CO-burning mode with two relativelydense beds of catalyst connected by a dilute phase transport conduitwhere the transport conduit is not vertical, but passes the catalyst andgas laterally.

From the foregoing it can be seen that the practice of the presentinvention permits petroleum refiners to minimize the capital cost of newunits, and minimize the amount of catalyst required, both for theinitial loading and for daily addition, by incorporating the presentinvention into the regenerator design. The present invention may be usedto advantage in the revamp of existing units to improve the regenerationprocess and to permit an increase in the processing capacity of theregenerator.

We claim as our invention:
 1. A regeneration apparatus for regeneratingspent catalyst with a regeneration gas supplied to said apparatus viainlets consisting essentially of a primary and secondary fluidizing gasinlet which comprises a combination of:(a) a vertical mixer zone havingat the lower portion thereof a spent catalyst inlet, a regeneratedcatalyst inlet and a secondary fluidizing gas inlet for passage of aminor portion of said fluidizing gas, and having at the upper portionthereof an outlet for passage of a mixture of spent and regeneratedcatalyst; (b) a relatively dense-phase fluidized bed catalyst chamberhaving at least twice the diameter of said mixer zone and having at thebottom thereof an inlet means for receiving a mixture of spent andregenerated catalyst, and having a regeneratedcatalyst and spentregeneration gas outlet means at the top portion of said chamber forremoval of regenerated catalyst and spent regeneration gas in admixturefrom said chamber; (c) a frusto-conical transfer section possessing aprimary regeneration gas inlet and a relatively dense-phase fluidizedbed in open communication with said mixer zone and said chamberdischarging upwardly from said mixer zone and upwardly into saidchamber, said frusto-conical transfer section having the smallesthorizontal cross section perimeter adjacent to said mixer zone outletand wherein said transfer section possesses multiple perforations at thelowermost portion of the same for passage of a major portion of saidregeneration gas, thereby constituting said primary gas inlet, locatedabove said spent catalyst, said regenerated catalyst and said fluidizinggas inlets of said mixer zone; (d) a regenerated catalyst receiving zonein communication with said chamber outlet means, said receiving zonecontaining a spent-regeneration-gas outlet means for the withdrawal ofspent regeneration gas from said regeneration apparatus, an upperregenerated-catalyst outlet means for the withdrawal from saidregeneration apparatus of a portion of regenerated catalyst and aregenerated-catalyst recycle conduit connected to said mixer zone bymeans of said regenerated catalyst inlet for the passage of a portion ofsaid regenerated catalyst from said receiving zone through said conduitand said inlet to said mixer zone.